1. Field of the Invention
This invention is in the field of applying porcelain enamel coatings and, more specifically, is directed to a method of treating a ferrous surface for the reception of a porcelain enamel coating, the pre-treating being done by means of a soft glass composition preferably in fritted form which reacts with the underlying ferrous surface to etch the same and provide a means for bonding the enamel coating to the underlying surface.
2. Description of the Prior Art
Ever since sheet steel has been provided with procelain enamel coatings, it has been considered necessary to subject the steel to a varied series of plating and preparation steps in order to improve the adhesion and the appearance of the porcelain enamel. The only exception to this rule has been in connection with very thin hollow ware of a very inexpensive nature where surface preparation was kept at a minimum. In any quality work, however, it was always considered necessary to prepare the surface by chemical and mechanical procedures before the procelain enamel frit was applied. A typical series of treatment steps for a steel sheet involved first repeatedly dipping the sheet in an alkaline cleaner at temperatures ranging from about 140.degree. to 212.degree. F. (60.degree. to 100.degree. C.). This was followed by a warm rinse with water at about 120.degree. to 140.degree. F. (49.degree. to 60.degree. C.). Following the warm rinse, there was usually a cold rinse with water at room temperature. Then came a pickling step with which the washed material was treated with a solution of 5 to 10% sulfuric acid at temperatures ranging from about 150.degree. to 160.degree. F. (66.degree. to 71.degree. C.). Following the pickling step, the material was again rinsed with water containing some small amount of sulfuric acid at room temperature. Following the cold rinsing step, a flash nickel deposition took place using temperatures of 130.degree. to 180.degree. F. (60.degree. to 82.degree. C.) to deposit about 0.02 to 0.6 grams of nickel per square foot. Following the nickel deposition, the steel was again cold rinsed with water and a small amount of sulfuric acid to prevent formation of ferric iron. Finally, the surface was neutralized with agents such as sodium carbonate and borax at temperatures of about 120.degree. to 130.degree. F. (49.degree. to 54.degree. C.). The equipment required for this extensive treatment necessitated substantial capital investment and the time and labor involved provided a substantial portion of the cost of the enameled product.
There have been a few disclosures in the prior art which sought to avoid the expensive and time-consuming pickling and nickel coating steps in enameling steel but to our knowledge they had little or no success commercially. One such disclosure appears in the expired Zimmerman U.S. Pat. No. 2,828,218 which described a frit which was applied as a ground coat to a metallic surface at an application weight of 1/8 to 3/8 ounces per square foot dry weight. The frit composition included materials such as flint, feldspar, dehydrated borax, soda ash, sodium nitrate, fluorospar, calcspar and red iron oxide. A conventional porcelain enamel cover coat was applied over the fired ground coat.
An improvement to this type of process is found in U.S. Pat. No. 2,786,782 issued to Zimmerman et al. In this patent, Zimmerman et al suggested adding black iron oxide (magnetite) and zinc oxide to frit compositions in order to improve the adherence. This material was applied in the conventional manner with mill additions as a ground coat, and after firing of the ground coat, a conventional porcelain enamel frit was applied and fired. Typically, the ground coat of this patent had from 5 to 13% magnetite, 2 to 7% zinc oxide, 35 to 42% silica, 20 to 25% boric oxide, 11 to 22% of one or more alkali metal oxides, 4 to 6% calcium oxide and 3 to 5% alumina.
A third reference which refers to the possibility of eliminating surface preparation such as etching or metal plating is U.S. Pat. No. 2,864,721 issued to King et al. This disclosure deals with a ground coat produced by milling a slip directly from raw batch enameling materials, without fritting, and applying the slip to the metallic article at a dry weight of 8 to 17 grams per square foot. The preferred ground coat included from 1 to 15% magnetite, 0 to 9% zinc oxide, 30 to 45% silica, 17 to 28% boric oxide, 11 to 22% of an alkali metal oxide, 3 to 8% calcium oxide, and 0 to 7% alumina.
In recent times, the technique of applying a ground coat and a cover coat of procelain enamel by means of electrostatic deposition has become more popular. Particular emphasis has been placed upon providing a two coat-one fire system for steels such as cold rolled steel. Examples of such current practice will be found in publications such as "Rationalization in the Enamelling Industry With Electrostatic Dry Procedures", a paper given at the International Congress in Vitreous Enamelling, in October, 1975; an article in "Iron Age" for December 15, 1975 entitled "Enamelers Place Hopes in Dry Frit Spraying", pages 49 to 52; and the article entitled "Powder: A Shot in the Arm for Porcelain Enameling" appearing in Industrial Finishing, January, 1976, pages 29 to 31. In each of these articles, however, the two coat-one fire system was applied to regularly pickled workpieces.